1. Field of the Invention
This invention relates to a method and apparatus for testing the rotating memory devices. More particularly, this invention relates to a method and apparatus for determining when contacts or near contacts between a read/write transducer and a memory medium of a rotating memory device occur based on the amplitude modulation of the playback signal from the read/write transducer.
2. Description of the Prior Art
In rotating memories (disk drives), the read/write transducer typically flies tenths of microns from the disk surface. Because the read/write transducer flies so close to the disk surface, it is desirable to be able to measure the smoothness of the disk surface to determine the ability of the read/write transducer to fly without touching the surface of the disk.
Identification of transducer to disk contacts or near contacts is crucial to overall disk drive reliability. Disk surface terrain is a result of manufacturing defects or contamination. The read/write transducer flies on a cushion of air (air bearing) over the disk surface. As the read/write transducer encounters a defect or dust particle on the disk surface, it will attempt to follow the air cushion and fly over a defect--subject to the mechanical response time of the transducer. If the mechanical response of the transducer is not fast enough, the read/write transducer will make contact with the imperfection on the disk surface. This results in either immediate head crash or the gradual damaging of the disk surface. The gradual damage occurs from particle spreading caused by slight impacts between transducer and the contaminate or defect.
Another failure mode occurs when a read/write transducer contacts the disk surface immediately after flying over an imperfection on the disk surface which causes a disruption in the air cushion supporting the read/write transducer. After repeated transducer-to-disk contacts, a depression in the disk surface begins to form and eventually a head crash may occur.
To ensure a reliable disk drive, it is necessary to identify disks which have topographical defects on their surface. Reliability problems are caused from defects in surface topography and not defects in magnetic material. Testing methods for reading and writing patterns to the magnetic material on the disk is of limited value because this testing indicates defects in the magnetic material and provides no information regarding the smoothness of the disk surface.
In the prior art, disk surface integrity has been measured using a separate transducer to determine if the disk surface is flat and smooth enough for a read/write transducer to fly without touching the surface. This special transducer typically includes a piezoelectric sensor for converting mechanical motion to an electrical signal.
There are several disadvantages associated with the prior art method. First, although it measures the disk surface, it does not measure the interaction of the read/write transducer with the disk surface. The actual read/write transducer is shaped differently than the special transducer, has different mechanical forces acting on it, and flies at a different height above the disk surface. Because of these differences between the special transducer and the actual read/write transducer, there is a difference in how each transducer responds to varying disk surface topography.
Second, the prior art method requires the disk drive to be unsealed so that a special transducer can be used. Unsealing the disk drive introduces the possibility of contamination of the disk surface.
Third, because a clean room is required to prevent contamination of the disks, no on-site integrity testing is possible. On-site testing is desirable for detecting damage or contamination introduced during the assembly and shipping processes.